Find the missing number in the series: 144, 441, 169, 961, 196, 1681, 225, ___

Key insight: the terms are perfect squares arranged in pairs where each base number is the digit-reversal of the previous base.

• 12^2 = 144 and 21^2 = 441

• 13^2 = 169 and 31^2 = 961

• 14^2 = 196 and 41^2 = 1681

• 15^2 = 225 and 51^2 = 2601

Therefore the missing number is 2601 (which is 51^2).